IANAL either, nor did i stay at a holiday in express, but i fail to see how verbal communication alone can be official. That being said, I would not make multiple simultaneous offers. But I do not see why you can not say in an offer, "I offer you this", this offer is good for 48 hours, or unless another offer I have made is accepted.
As a suebeelin stated...waiting almost 2 weeks??? That is the seller hoping for something better to come in, so why can't a buyer look for something better?
In today's world, we get emails on our phones in seconds. If a seller can not make a decision in a day or two, that is their problem.
Realistically, if I put in an offer, and here nothing for weeks, I would forget about it. If I get a call a year later telling me it was accepted, i have a strong suspicion I am under no obligation.....
It is probably in bad taste to rescind an offer in a short time frame, and I would think you can really avoid making multiple offers most of the time. If a contract pops up, and is that good that it will not be there in two days, you probably have to offer what the seller is asking for and no less. And if a broker gets that, I think they act on behalf of the seller, and put sale pending.
All this being said, I would simply state, this offer stands for 48 hours, and problem solved.